Method for silver plating

ABSTRACT

The aim of the present invention is to provide a method for silver plating that does not comprise the step of forming the nickel-underlayer and that can form a silver-plated layer having sufficient adherence directly on the difficult-to-plate substrates with the use of a halide-free plating bath under a good working environment. The present invention provides a silver plating method onto a substrate on which an oxide layer inhibiting adherence of a plated layer is prone to form, comprising at least the following steps of; (A) degreasing the substrate, (B) removing the oxide layer with a strongly acidic solution from the substrate, and (C) silver plating onto the substrate, without a step of nickel or nickel alloy strike plating in advance, utilizing a phosphines-containing acidic silver plating bath which essentially does not contain halide ion or cyanide ion.

FIELD OF THE INVENTION

The present invention relates to a method of non-cyanide silver electroplating and in particular, to a silver plating method which provides good adherence onto the so-called difficult to plate metals by utilizing an acidic plating bath.

BACKGROUND OF THE INVENTION

Silver has industrially applicable properties such as good electroconductivity along with good solderability, and is thus widely utilized for manufacturing of electrical parts. Furthermore, its aesthetic appeal has been widely appreciated for decorative purposes. Moreover, owing to its ductility, it can be utilized to prevent seizure of screws or to improve lubricity and wear/abrasion resistance of sliding parts.

To prevent seizure of screws, or to improve lubricity and abrasion/wear resistance of sliding parts, silver plating is often required onto so called difficult-to-plate metallic substrates. Such substrates are prone to form an oxide layer on their surfaces, or to cause displacement deposition, or the combination of the both. As these factors leads to poor adhesion, they are referred to as difficult-to-plate metallic substrates. Such substrates include aluminum (alloys), magnesium (alloys), tin (alloys), stainless-steel, titanium (alloys) and die cast zinc alloys.

As a method for plating onto the difficult-to-plate metallic substrates with good adherence, nickel strike plating is commonly utilized. The method comprises, immediately after removing an oxide layer from a substrate with an acid, forming a very thin nickel layer on the substrate using a nickel plating solution which contains a large amount of chloride, and then forming a desired plated layer thereon.

The method utilizing nickel strike plating has been widely practiced as general procedure with some modifications over the years.

For instance, Japanese unexamined patent publication No. 2005-133169 discloses a method for producing a silver-plated stainless steel strip for moving contacts comprising forming a nickel underlayer onto a stainless steel base metal surface, and then forming a silver- or silver alloy-plated layer thereon,

Furthermore, Japanese unexamined patent publication No. 2002-237312 discloses a method for producing a silver or silver-alloy plated metal separator for solid electrolyte batteries comprising silver- or silver alloy-plating onto a stainless steel having a nickel plated layer as an underlayer.

However, elimination of nickel-strike plating step affords reduction in manufacturing cost. Furthermore, for optimization of the favorable properties of a silver plated layer, the presence of nickel underlayer is sometimes not preferable. For that purpose, Japanese unexamined patent publication No. 2002-121693 discloses a method and bath for silver plating onto stainless steel without nickel strike plating, the method utilizing a plating bath containing halide ion with a pH value of −1.0˜2.0.

SUMMARY OF THE INVENTION

It has been known that fair adherence can be achieved without a presence of nickel underlayer by utilizing a silver plating bath which contains high concentration of halide ion, but industrial application of such bath entails some problems. As an iodide bath is commonly utilized as the halide bath, the problems will be hereafter explained specifically in connection with the iodide bath.

(1) With high concentration of halide ion, silver forms complex which dissolves easily whereas with low halogen concentration, it exhibits poor solubility and such property of silver halide is utilized for detection of residual halogen. Since precipitation of silver is generated in a low-concentration iodide bath, use of a high-concentration iodide bath is preferable but doing so may create problems such as corrosion of equipment or wastewater management.

(2) Moreover, in a rinse process, because the plating solution attached on the surface of the object to be plated is diluted with rinse water and the concentration of iodide ion becomes low, precipitation of silver iodide, which is very difficult to remove, is formed on the surface.

(3) Silver-plated layer deposited from an iodide bath exhibits low ductility and it is prone to cracking.

(4) In an iodide bath, iodine is generated through the oxidization of iodide ion on the anode and that has, even if slightly generated, negative effect on the plated layer which causes poor adherence and coarsening. Furthermore, when significant amount of iodine is generated, iodine may be released as a toxic gas. In particular, since the generation of toxic gas becomes significant when an insoluble anode is used, use of the insoluble anode becomes unacceptable.

To optimize the properties of silver-plated layer, there has been a demand for a method of silver plating that does not comprise the step of forming a nickel plated layer between the difficult-to-plate substrate and the silver plated layer. However, there is much to be improved to meet industrial requirements.

Thus the object of the present invention is to provide a method for silver plating that does not comprise the step of forming the nickel-underlayer and that can form a silver-plated layer having sufficient adherence directly on the difficult-to-plate substrates with the use of a halide-free plating bath under a good working environment.

The present inventors have found that a fine silver-plated layer with good adherence can be formed onto the difficult-to-plate substrate by utilizing a silver plating method that can prevent formation of an oxide layer, the method comprising silver plating onto the substrate using an acidic silver plating bath which contains phosphines as a complexing agent and substantially does not contain halide ion, preceded by degreasing the substrate and then removing the oxide layer formed on the substrate surface from the substrate with a strongly acidic solution.

Accordingly, in one aspect, the present invention is a silver plating method onto a substrate on which an oxide layer that inhibits adherence of a plated layer is prone to form, comprising at least the following steps of;

(A) degreasing the substrate,

(B) removing the oxide layer with a strongly acidic solution from the substrate, and(

(C) silver plating onto the substrate, without a step of nickel or nickel alloy strike plating in advance, utilizing a phosphines-containing acidic silver plating bath which essentially does not contain halide ion or cyanide ion.

In one embodiment of the present invention, the method further comprises (D) silver plating onto the substrate utilizing a sulfonic acid-containing acidic silver plating bath preceded by step (C).

In another embodiment of the present invention, the phosphines-containing acidic silver plating bath in step (C) further contains sulfonate ion.

In yet another embodiment of the invention, the silver plating baths in steps (C) and (D) both have a pH value of less than or equal to 3.

In yet another embodiment of the present invention, the silver plating bath in step (C) contains one or more of the phosphines represented by the general formula (1):

wherein X1, X2 and X3, which may be the same or different, each representing a hydrogen atom, a substituted or unsubstituted C1˜C10 alkyl group, or a substituted or unsubstituted benzene ring, the substituents for the substituted alkyl group or the substituted benzene ring being one or more selected from the group consisting of a hydroxyl group, a carboxyl group, a sulfonic group and an amino group, provided that not all of X1, X2 and X3 being hydrogen atoms simultaneously.

In yet another embodiment of the invention, the phosphines are lower alkylphosphine represented by the general formula (2).

wherein Y1, Y2 and Y3, which may be the same or different, each representing an unsubstituted C1˜C3 alkyl group or a C1˜C3 alkyl group substituted with one or more substituents selected from the group consisting of a hydroxyl group, a carboxyl group, a sulfonic group and an amino group.

In yet another embodiment of the invention, the strongly acidic solution in step (B) contains more than or equal to 10 wt % of acid.

In yet another embodiment of the invention, the strongly acidic solution in step (B) has a pH value of less than or equal to 2.

In yet another embodiment of the invention, the strongly acidic solution in step (B) essentially does not contain halide ion.

In yet another embodiment of the invention, the strongly acidic solution in step (B) contains sulfonic acid.

In yet another embodiment of the invention, the above mentioned substrate is made of magnesium, aluminum, titanium, chromium, nickel cobalt, zinc and tin, or an alloy which at least contains one or more metals selected from the group consisting of these metals.

In yet another embodiment of the invention, the substrate is made of a chromium-containing alloy.

In yet another embodiment of the invention, the substrate is made of a stainless steel.

In yet another embodiment of the invention, the substrate is made of an austenitic stainless steel.

In yet another embodiment of the invention, the substrate is made of titanium.

In yet another embodiment of the invention, the substrate is made of tin or tin-alloy.

In yet another embodiment of the invention, the above mentioned substrate is an alloy of tin and at least one or more metals selected from copper, zinc, silver, indium, gold, lead and bismuth.

In yet another embodiment of the invention, an insoluble anode is used in the silver plating step (C) and/or (D).

In yet another embodiment of the invention, the insoluble anode is selected from the group consisting of a carbon anode, a platinum anode, a platinum-coated titanium anode, an oxidized ruthenium-coated anode and an oxidized iridium-coated anode.

In yet another embodiment of the invention, the above mentioned insoluble anode comprises an uppermost layer made of one or more material(s) selected from the group consisting of spinel, garnet, glass, and perovskite.

According to the cyanide-free silver plating method provided by the present invention, oxidation of the surface of the difficult-to-plate substrate can be prevented and direct silver plating onto such substrate with good adherence can be achieved, by forming a silver-plated layer using the acidic silver plating solution containing phosphines as a reducing compound immediately after removing the oxide layer from the substrate with an acidic solution.

Furthermore, problems associated with silver plating baths such as (1) corrosion of plating equipments and wastewater management, (2) generation of silver compound on the substrate surface during water-rinsing process, (3) cracking-prone plated layer, (4) deterioration of plating bath caused by the generation of iodine and (5) incompatibility with insoluble anodes, can all be resolved.

Moreover, by utilizing the solution which contains sulfonic acid as a strongly acidic solution for removing the oxide layer on the difficult-to-plate substrate, it also becomes possible to eliminate halide ion from the removal process and as a result, elimination of halide ion from whole process can be essentially achieved.

PREFERRED EMBODIMENTS OF THE INVENTION

Following is the detailed descriptions of the silver plating method provided by the present invention.

The present invention is a silver plating method onto a substrate on which an oxide layer that inhibits adherence of a plating that inhibits adherence of a plated layer is prone to form, comprising at least the following steps of;

(A) degreasing the substrate,

(B) removing the oxide layer with a strongly acidic solution from the substrate, and

(C) silver plating onto the substrate, without a step of nickel or nickel alloy strike plating in advance, utilizing a phosphines-containing acidic silver plating bath which essentially does not contain halide ion or cyanide ion.

Therefore one of the advantages provided by the present invention is that it affords the elimination of nickel or nickel-alloy strike plating step carried out in a case of the usual difficult-to-plate substrates, thereby silver plating can be applied directly following step (B). It is common that both the strongly acidic solution as well as the silver plating bath being provided in the form of aqueous solution.

By removing the oxide layer in step (B) and then plating with the acidic silver plating bath which contains phosphines, which are compounds with a strong reducing ability, the reformation of the oxide layer is prevented and thus plated layers having good adherence can be obtained even onto the substrate surface which is prone to the formation of the oxide layer.

It is preferable to conduct step (C) immediately after step (B), to prevent the reformation of the oxide layer on the substrate surface. For instance, time spent between lifting the object to be plated from the strongly acidic solution and immersing the object into the silver plating bath should be 5˜120 seconds, more preferably 5˜15 seconds including time for rinsing.

For the degreasing step (A), degreasing method can be selected from any methods known to those skilled in the art and include, but not limited to, acid degreasing, alkaline degreasing, solvent degreasing, emulsion degreasing, electrolytic degreasing, and machine degreasing. However, among them, acid degreasing is the most preferable for the purpose of minimizing pH fluctuation of the substrate surface, thereby improving adhesion.

According to the present invention, silver plating step can be carried out with the acidic silver plating bath which contains the above mentioned phosphines without any other plating bath. However, silver plating may be carried out by a flash silver plating with the phosphines-containing bath, followed by (D) a silver plating with the sulfonic acid-containing acidic bath exhibiting a higher deposition efficiency.

The requisite of the present invention is the utilization of an acidic plating solution as the silver plating baths for the above mentioned step (C) and (D). For the both steps, the acidic bath with a pH value of less than or equal to 3 is preferable and less than or equal to 2 is more preferable. As the acid to control the pH of the silver plating bath, halide free-acids such as organic sulfonic acids, sulfuric acid, phosphoric acid, fluoboric acid are preferable. Among them, organic sulfonic acids, in particular, methane sulfonic acid, alkanol sulfonic acid and phenolsulfonic acid are preferable from the viewpoint of solubility of silver ion and stability of the bath.

In the present invention, the silver plating bath for step (C) contains at least one or more of aliphatic or aromatic phosphines represented by the general formula (1):

wherein X1, X2 and X3, which may be the same or different, each representing a hydrogen atom, a substituted or unsubstituted C1˜C10 alkyl group, or a substituted or unsubstituted benzene ring, the substituents for the substituted alkyl group or the substituted benzene ring being one or more selected from the group consisting of a hydroxyl group, a carboxyl group, a sulfonic group and an amino group, provided that not all of X1, X2 and X3 being hydrogen atoms simultaneously.

In addition, the lower alkyl phosphines represented by the general formula (2) can be more preferably used.

wherein Y1, Y2 and Y3, which may be the same or different, each representing an unsubstituted C1˜C3 alkyl group or a C1˜C3 alkyl group substituted with one or more substituents selected from the group consisting of a hydroxyl group, a carboxyl group, a sulfonic group.

The preferable phosphines include, for example, unsubstituted alkyl phosphines in which each alkyl group is a methyl group, an ethyl group or a propyl group; and substituted alkyl phosphines in which each alkyl group is substituted by one or more substituents selected from the group consisting of a hydroxyl group, a carboxyl group, a sulfonic group, and an amino group. The substituted alkyl phosphines include hydroxy lower alkyl phosphines having a hydroxymethyl group, a hydroxyethyl group or a hydroxypropyl group; carboxy lower alkyl phosphines having a carboxymethyl group, a carboxyethyl group or a carboxypropyl group; sulfo lower alkyl phosphines having a sulfomethyl group, a sulfoethyl group or a sulfopropyl group; and amino lower alkyl phosphines having an aminomethyl group, an aminoethyl group or an aminopropyl group.

Among phosphines described above, tris(hydroxy lower alkyl)phosphines, in which one hydrogen atom on each lower alkyl group is substituted by a hydroxyl group to form each hydroxy lower alkyl group selected from the group consisting of a hydroxymethyl group, a hydroxyethyl group and a hydroxypropyl group can be more preferably used in terms of cost and stability. Tris(3-hydroxypropyl)phosphine can be most preferably used.

The phosphines-containing acidic silver plating bath for step (C) may also contain sulfonate ion. Moreover, a sulfonic acid bath can be preferably used as the bath for step (D) in light of plating bath stability, appearance of electrodeposited layer and electrical properties such as surface resistance.

Although any of the aliphatic or aromatic sulfonic acids may be preferably used, aliphatic sulfonic acids are more preferably used.

Among aliphatic sulphonic acids, aliphatic acids such as alkane sulfonic acids or alkanol sulfonic acids can be preferably used. As the above mentioned alkane sulfonic acids, for instance, methanesulfonic acid, ethanesulfonic acid, 1-propanesulfonic acid, 2-propanesulfonic acid, 1-buthanesulfonic acid, 2-buthanesulfonic acid and pentanesulfonic acid can be listed. As the above mentioned alkanolsulfonic acids, for instance, 2-hydroxyethane-1-sulfonic acid (isethionic acid), 2-hydroxypropane-1-sulfonic acid (2-propanolsulfonic acid), 2-hydroxybuthane-1-sulfonic acid, 2-hydroxypentane-1-sulfonic acid as well as 1-hydroxypropane-2-sulfonic acid, 3-hydroxypropane-1-sulfonic acid, 4-hydroxybuthane-1-sulfonic acid and 2-hydroxyhexane-1-sulfonic acid can be listed.

In the present invention, the solution containing more than or equal to 10 wt % of acid is preferably used for step (B) to remove the oxide layer. However, as corrosion of the equipments or adverse effect to working environment are expected when the concentration of the acid is excessively high, it is preferable that wt % of the acid kept less than or equal to 50%. Typically, the acid concentration is kept at 10˜50 wt % and more typically, it is kept at 10˜20 wt %.

Moreover, pH of the corresponding solution is kept preferably less than or equal to 2, and more preferably less than or equal to 1 considering removing ability of the oxide layer. Depending on the type of the substrate and the condition of the oxide layer, the type of acid can be selected from the known acid in the plain or mixed form. Nitric acid, sulfuric acid, phosphoric acid, hydrochloric acid, hydrofluoric acid as well as the above mentioned various sulfonic acids can be preferably used. However, to prevent corrosion of the equipments and drag-in to the silver plating solution in the following silver plating step, it is preferable not to use the acid which contains halide ion such as hydrochloric acid or hydrofluoric acid and thus the acidic solution which contains sulfonic acid as its main component is preferably used for the removal of the oxide layer.

The substrates on which an oxide layer that inhibits adherence of a plated layer is prone to form and to which the present invention can be suitably applied include magnesium, aluminum, titanium, chromium, nickel cobalt, zinc, tin and silicon, and an alloy which at least contains one or more metals selected from the group consisting of these metals.

Those expressed as the substrate here include not only the bulk metals (or alloys) but also plated layers.

Examples of the substrate include magnesium cast alloys containing aluminum or zinc, aluminum bronze cast alloys, silzin bronze cast alloys, aluminum cast alloys for bearings, stainless steels, nickel-phosphorous plating films, aluminum or copper-containing zinc die cast alloys, zinc alloy-plated layer such as zinc-iron or zinc-nickel, tin or tin alloy plated layer which contains copper, zinc, silver, indium, gold, lead or bismuth.

Among these substrates, the direct silver plating according to the present invention can be applied more preferably to stainless steels and most preferably to austenitic stainless steels.

According to the present invention, silver plating can be carried out by separating cathode and anode compartments with the ion-exchange membrane in either or both of (C) phosphine-containing acidic silver plating bath substantially containing neither halide ion nor cyanide ion and (D) sulfonic acid-containing acidic silver plating bath.

Although the ion-exchange membrane method can be preferably used for both baths (C) and (D), it can be most suitably used for bath (C).

Although either a cationic ion-exchange membrane or a anionic ion-exchange membrane can be used as the ion-exchange membrane, the anionic ion-exchange membrane can be more suitably used. By separating the cathode and anode compartments with the anionic ion-exchange membrane, decomposition of the additives such as a complexing agent, a leveler or a brightener which may be added to the plating bath, and the negative effect on the plated layer by the compound generated as the result of consumption or decomposition of the additives, can be prevented. Moreover, increase in silver concentration in the bath due to the use of silver anode can be prevented and control of metal concentration in the bath can be done easily.

Furthermore, according to the silver plating method of the invention, an insoluble anode can be used as an anode. The anode made of known materials such as carbon, platinum, platinum-coated titanium, ruthenium oxide-coated titanium or carbon, or iridium oxide-coated titanium or carbon may be utilized. Preferably, the insoluble anode may comprise an uppermost layer made of one or more selected from the group consisting of spinel, garnet, glass, and perovskite.

Thus, the soluble silver anode, the above mentioned insoluble anode or the combination of both, can be used as the anode.

The silver plating method of the present invention utilizing the acidic bath comprises, although not limited to, the steps of (A) degreasing the substrate, (B) removing the oxide layer with a strongly acidic solution from the substrate, and (C) silver plating onto the substrate utilizing a phosphines-containing acidic silver plating bath which essentially does not contain halide ion or cyanide ion. Following the silver plating step (C), step (D) of silver plating with the sulfonic acid-containing acidic silver plating bath may be applied. Normally, water-rinsing is conducted between each step.

The conditions for step (C) is that bath temperature should be generally kept at 10˜50 degrees C., more preferably at 20˜35 degrees C.

Preferable current density is 0.5˜5 A/dm2, and more preferably 2-3 A/dm2. Plating time should be 10˜300 seconds and more preferably 20˜100 seconds.

The conditions for step (D) is that bath temperature should be generally kept at 10˜50 degrees C., and more preferably at 15˜40 degrees C. Preferable current density is 0.1˜10 A/dm2, and more preferably 0.5˜5 A/dm2. Plating time can be set depending on the plating thickness required.

EXAMPLES

Following is the detailed descriptions of the present invention with regard to the actual examples. These examples are meant to be the illustrative of the invention and not limiting the application of the invention.

Each plating process was evaluated from the aspect of adhesiveness of the plated layer. Plating adherence was evaluated through the bending test. The bending test (90 degrees×3 times) was conducted according to JIS-H8504 standard and the extent of peeling was observed.

Daiwa Fine Chemical general purpose acidic cleaner AC-100 was used for degreasing.

Example 1

Stainless steel (SUS304) was selected as a substrate. (A) acid degreasing, →(B) removal of an oxide layer with a strongly acidic solution→(C) acidic silver plating drying was carried out in this order. Water-rinsing step was conducted between each step.

The composition of the solution used in each step is shown below. Time spent from when the object to be plated was taken out of the strongly acidic solution until it was immersed into the silver plating bath was 15 seconds, including 5 seconds for immersing the object into the rinse water.

strongly acidic solution methanesulfonic acid 150 g/L (15 wt %) temperature 25 degrees C. silver plating bath tris(3-hydroxypropyl)phosphine 20 g/L sulfuric acid 40 g/L silver sulfate (as silver) 3 g/L pH −0.4 type of anode carbon type of ion-exchange membrane anion-exchange membrane temperature 25 degrees C. current density 3A/dm3 plating time 90 seconds

The silver-plated material thus obtained was subjected to the bending test and no sign of cracking or peeling was observed, thereby exhibiting excellent ductility and adherence.

Example 2˜20

Unless otherwise stated in Table 1, silver plating for each example was conducted according to the conditions stated in Example 1. The results of the bending tests are also provided in table 1.

If no cracking or peeling was observed after bending three consecutive times and thus good ductility as well as adherence were exhibited, such result was evaluated as grade “excellent”. If slight sign of cracking or peeling was observed after bending three consecutive times, grade “good” if slight sign of cracking or peeling was observed after bending twice, such result was evaluated as grade “not bad”. If obvious sign of cracking or peeling was observed before the completion of the second bending, such result was evaluated as “bad”.

TABLE 1 No Modification from Example 1 Result Ex: 2 An aluminum-containing magnesium alloy was used Good as the substrate. Ex: 3 A zinc-containing magnesium alloy was used as the Good substrate. Ex: 4 An aluminum-bronze was used as the substrate. Excellent Ex: 5 A silzin-bronze was used as the substrate. Excellent Ex: 6 A stainless steel (SUS 316 ) was used as the Excellent substrate. Ex: 7 The substrate (in example 1) plated with a zinc- Good nickel alloy layer was tested. Ex: 8 The substrate (in example 1) plated with a copper- Excellent tin alloy layer was tested. Ex: 9 A silver plating bath with the following composition Excellent was used: tris(3-hydroxypropyl)phosphine 30 g/L methanesulfonic acid 50 g/L sulfuric acid 10 g/L silver methanesulfonate (as silver) 2 g/L pH −0.1 Ex: 10 A silver plating bath with the following composition Excellent was used tris(hydroxymethyl)phosphine 30 g/L phosphoric acid 70 g/L silver oxide (as silver) 1 g/L pH −0.1 Ex: 11 A silver plating bath with the following composition Excellent was used tris(2-hydroxyethyl)phosphine 60 g/L mehanesulfonic acid 30 g/L silver methanesulfonate (as silver) 5 g/L pH −0.5 Ex: 12 A silver plating bath with the following composition Excellent was used tris(3-aminopropyl)phosphine 20 g/L sulfuric acid 50 g/L silver sulfate (as silver) 10 g/L pH −0.3 Ex: 13 A strongly acidic solution with the following Excellent composition was used sulfuric acid 300 g/L (30 wt %) pH 0.8 Ex: 14 A strongly acidic solution with the following Not bad composition was used methanesulfonic acid 1 g/L (0.1 wt %) sodium hydrate 0.3 g/L pH 5 Ex: 15 The time spent before the immersion into the silver Not bad plating bath was 100 seconds in total, including 20 seconds for water-rinsing. Ex: 16 following step (C), the extra silver plated-layer was Good formed with the following silver plating bath methanesulfonic acid 50 g/L silver methanesulfonate (as silver) 20 g/L pH □0.3 bath temperature 25 degrees C. Ex: 17 A titanium anode covered with platinum uppermost Good layer was used as the anode. Ex: 18 An aluminum-containing magnesium alloy was used Excellent as the substrate. Ex: 19 A stainless steel (SUS410) was used as the Good substrate. Ex: 20 A carbon electrode covered with spinel uppermost Excellent layer was used as an anode 

1. A silver plating method onto a substrate on which an oxide layer that inhibits adherence of a plated layer is prone to form, comprising at least; (A) degreasing the substrate, (B) removing the oxide layer with a strongly acidic solution from the substrate, and (C) silver plating onto the substrate, without a step of nickel or nickel alloy strike plating in advance, utilizing a phosphines-containing acidic silver plating bath which essentially does not contain halide ion or cyanide ion.
 2. The silver plating method according to claim 1, wherein the method further comprises (D) silver plating onto the substrate utilizing a sulfonic acid-containing acidic silver plating bath preceded by step (C).
 3. The silver plating method according to claim 1, wherein the phosphines-containing acidic silver plating bath in (C) further contains sulfonate ion.
 4. The silver plating method according to claim 2, wherein the silver plating baths in (C) and (D) both have a pH value of less than or equal to
 3. 5. The silver plating method according to claim 1, wherein the silver plating bath in (C) contains one or more of the phosphines represented by the general formula (1):

wherein X₁, X₂ and X3, which may be the same or different, each representing a hydrogen atom, a substituted or unsubstituted C1˜C10 alkyl group, or a substituted or unsubstituted benzene ring, the substituents for the substituted alkyl group or the substituted benzene ring being one or more selected from the group consisting of a hydroxyl group, a carboxyl group, a sulfonic group and an amino group, provided that not all of X₁, X₂ and X₃ being hydrogen atoms simultaneously.
 6. The silver plating method according to claim 5 wherein the phosphines are lower alkylphosphine represented by the general formula (2):

wherein Y₁, Y₂ and Y₃, which may be the same or different, each representing an unsubstituted C1˜C3 alkyl group or a C1˜C3 alkyl group substituted with one or more substituents selected from the group consisting of a hydroxyl group, a carboxyl group, a sulfonic group and an amino group.
 7. The silver plating method according to claim 1, wherein the strongly acidic solution in (B) contains more than or equal to 10 wt % of acid.
 8. The silver plating method according to claim 1, wherein the strongly acidic solution in step (B) has a pH value of less than or equal to
 2. 9. The silver plating method according to claim 1, wherein the strongly acidic solution in (B) essentially does not contain halide ion.
 10. The silver plating method according to claim 1, wherein the strongly acidic solution in (B) contains sulfonic acid.
 11. The silver plating method according to claim 1, wherein the above mentioned substrate is made of magnesium, aluminum, titanium, chromium, nickel cobalt, zinc and tin, or an alloy which at least contains one or more metals selected from the group consisting of these metals.
 12. The silver plating method according to claim 11, wherein the substrate is made of a chromium-containing alloy.
 13. The silver plating method according to claim 12, wherein the substrate is made of a stainless steel.
 14. The silver plating method according to claim 13, wherein the substrate is made of an austenitic stainless steel.
 15. The silver plating method according to claim 11, wherein the substrate is made of titanium.
 16. The silver plating method according to claim 11, wherein the substrate is made of tin or tin-alloy.
 17. The silver plating method according to claim 11, wherein the above mentioned substrate is an alloy of tin and at least one or more metals selected from copper, zinc, silver, indium, gold, lead and bismuth.
 18. The silver plating method according to claim 1 wherein an insoluble anode is used in at least one of (C) and (D).
 19. The silver plating method according to claim 18 wherein the insoluble anode is selected from the group consisting of a carbon anode, a platinum anode, a platinum-coated titanium anode, an oxidized ruthenium-coated anode and an oxidized iridium-coated anode.
 20. The silver plating method according to claim 18 wherein the uppermost layer of the above mentioned insoluble anode comprises an uppermost layer made of one or more material(s) selected from the group consisting of spinel, garnet, glass, and perovskite.
 21. The silver plating method according to claim 2 wherein an insoluble anode is used in at least one of (C) and (D).
 22. The silver plating method according to claim 21 wherein the insoluble anode is selected from the group consisting of a carbon anode, a platinum anode, a platinum-coated titanium anode, an oxidized ruthenium-coated anode and an oxidized iridium-coated anode.
 23. The silver plating method according to claim 21 wherein the uppermost layer of the above mentioned insoluble anode comprises an uppermost layer made of one or more material(s) selected from the group consisting of spinel, garnet, glass, and perovskite. 